1. Field of the Invention
The invention relates to a sheet detecting apparatus for detecting the presence or absence of a sheet passing on a conveying path by a light emitter and a light receiver. The sheet detecting apparatus is utilized in an image forming apparatus such as a printer, an original conveying apparatus and a paper post-treating apparatus.
2. Related Background Art
A conventional sheet detecting apparatus shown in FIG. 9 of the accompanying drawings, as described, for example, in Japanese Patent Application Laid-open No. 2002-267767, has been comprised of a light emitter 510 and a light receiver 500 disposed in opposed relationship with each other with a sheet conveying path 530 interposed therebetween, a CPU 600 for controlling the state of the sheet detecting apparatus, a D/A converter 602 for converting a control signal from the CPU 600 into an analog signal, a voltage-current converting circuit (V/I converting circuit) 603 for converting an output from the D/A converter 602 into a current and generating a driving current for causing the light emitter 510 to emit light, a current-voltage converting circuit (I/V converting circuit) 604 for converting a photoelectric current generated by the light receiver into a voltage, and an A/D converter 605 for converting an output voltage from the I/V converting circuit into a digital signal and transmitting it to the CPU 600.
Such a conventional sheet detecting apparatus has detected the presence or absence of a sheet by the sheet intercepting light between the light emitter 510 and the light receiver 500 disposed in opposed relationship with each other as described in Japanese Patent Application Laid-open No. 2002-267767.
In the above-described conventional sheet detecting apparatus, however, there has been the problem that when dust such as paper powder adheres to the light emitter and the light receiver and an output value from the light receiver decreases, it is wrongly recognized that the sheet has passed, in spite of the sheet having not passed. If in this case, the sensitivity of the sheet detecting apparatus is set high in order to prevent the wrong recognition, there has arisen the problem that when a thin sheet is passed, light is transmitted therethrough and the sheet cannot be detected.
Also, there has been proposed a sheet detecting apparatus provided with an automatic correcting method of taking out and storing an output signal from a light receiver as the data of an initial state, comparing it with an output signal from the light receiver periodically measured, and adjusting the quantity of light of a light emitter in conformity with the comparison signal to thereby maintain it in an optimum state.
In such a sheet detecting apparatus however, it has been difficult to set the timing for monitoring the output signal from the light receiver, and this has led to the undesirable possibility that for example, automatic correction cannot be appropriately effected for such an unexpected cause as a sudden change in temperature or the temporary adherence of dust. Also, there has been the problem that the power supply to the sheet detecting apparatus is cut off during the renewal of correction data and the correction data so far backed up is destroyed.